Two new biologically active triterpenoidal saponins acylated with salicylic acid from Albizia adianthifolia

Abstract : Two new oleanane-type triterpene saponins, adianthifoliosides A (1) and B (2), were isolated from a 95% ethanolic extract of roots of Albizia adianthifolia. Their structures were elucidated mainly by using a combination of 600 MHz 1D and 2D NMR techniques (COSY, NOESY, TOCSY, HSQC, and HMBC) and by FABMS and HRESIMS. Compounds 1 and 2 were characterized as glycosides of acacic acid acylated by an o-hydroxybenzoyl unit. The crude saponin mixture (CSM), compounds 1 and 2 together with 3 and 4 (prosapogenins obtained from the mild alkaline hydrolysate of the CSM), were evaluated for immunomodulatory activity on the Jurkat T cell line and for hemolytic property against sheep erythrocytes. Compound 2 and, to a lesser extent, 1 and 3 were found to exhibit a dose-dependent immunomodulatory effect in the concentration range 10-2-10 µM, whereas 4 showed a lymphoproliferative activity in the same concentration range. Among the compounds tested, only 1 and 2 were found to be hemolytic. The genus Albizia comprises about 150 species widely distributed in the tropics, with the greatest diversity in Africa and Central and South America. 1 Albizia species have been reported to contain alkaloids, steroids, triter-penoid saponins, and flavonols. 2 Some saponins such as julibrosides J 1 , J 2 , and J 9 from Albizia julibrissin 3,4 possess various biological effects, such as inhibitory activity against the KB cancer cell line in vitro. In a previous contribution, we reported the isolation and structure determination of two prosapogenins (3 and 4) isolated from the butanol extract of the mild alkaline hydrolysate of the crude saponin fraction of the roots of Albizia adianthifolia (Schumach.) W. F. Wight (Mimo-saceae). 5 The further investigation of the saponins of this plant, obtained as a complex mixture, afforded two new acylated triterpene saponins, adianthifoliosides A (1) and B (2), from the 95% ethanolic extract of the roots of A. adianthifolia. This paper deals with the isolation and structure elucidation of these new acylated triterpene glycosides (1 and 2) and the evaluation of the immuno-modulatory activity of 1-4 (Chart 1) and the crude saponin mixture (CSM) on Jurkat T cell proliferation (human T cell leukemia) and the hemolytic activity of 1-4 on sheep erythrocytes. Results and Discussion The 95% ethanolic extract of the roots of A. adianthifolia was purified by precipitation with diethyl ether, yielding a crude saponin mixture, which was then dialyzed for 2 days. The powder obtained was submitted to column chromatography over Sephadex LH-20 and was separated by repeated medium-pressure liquid chromatography (MPLC) over normal Si gel, yielding compounds 1 and 2. Their structures were elucidated mainly by 1D and 2D NMR spectroscopy (COSY, TOCSY, NOESY, HSQC, and HMBC) and by FABMS and HRESIMS. Compound 1 was obtained as an amorphous powder. The HRESI mass spectrum (positive-ion mode) exhibited a quasimolecular ion peak at m/z 1714.7490 [M + Na] + (calcd 1714.7465), consistent with a molecular formula of C 79 H 121-NO 38. Upon acid hydrolysis with 2 N TFA at 120 °C, 1 afforded the aglycon 6, which was identified as acacic acid lactone (the 21,28-lactone derivative of acacic acid obtained under the experimental conditions used), by comparison of its NMR data with literature values. 6 The native aglycon was characterized as acacic acid from the 2D NMR spectra from 1. The sugars obtained from the saponin hydrolysate were identified as glucose, fucose, rhamnose, arabinose, and xylose by comparison with authentic samples. However , the presence of an N-acetamido group [IR 1639 and 1570 cm-1 ; 1 H NMR δ H 2.13 (3H, s, MeCO) and δ H 8.91 (1H, d, J) 8.8 Hz, NH); 13 C NMR δ C 22.9 and 171.9], together with the 1 H and 13 C NMR data of the C-2 of the Glc 1 (δ H 4.40 and δ C 56.8), suggested the presence of one 2-(acetylamino)-2-deoxyglucose (Glc 1 NAc) unit. In addition, alkaline hydrolysis of 1 afforded salicylic acid (SA, o-hydroxybenzoic acid, 7), identified by comparison of its 1 H and 13 C NMR data with those reported, 7 and a prosapoge-nin whose spectroscopic NMR data were in good agreement with those of 4. 5 The above data suggested that 1 is a 21-acyl-3,28-bisdesmoside. This was confirmed by the observation of glycosylation-and acylation-induced shifts in the 13 C NMR spectrum at δ C 88.7 (downfield shift of C-3), δ C 76.5 (downfield shift of C-21), and δ C 174.4 (upfield shift of C-28). The 1 H NMR spectrum of 1 displayed signals for seven anomeric protons at δ H 6.06 (br s), 5.83 (d, J) 7.5 Hz), 5.77 (br s), 5.10 (d, J) 7.5 Hz), 4.91 (d, J) 7.5 Hz), 4.90 (d, J) 7.8 Hz), and 4.79 (d, J) 8.0 Hz), which correlated with the carbon signals at δ C 109.8, 94.9, 101.2, 104.7, 103.8, 105.6, and 102.5, respectively, in the HSQC spectrum. Starting from the anomeric proton of each sugar unit, all the protons within each spin system were delineated using COSY with the aid of TOCSY and NOESY spectra. After assignments of the protons, the 13 C NMR resonances of each sugar unit were identified by HSQC and further confirmed by HMBC. The COSY and TOCSY spectra
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Journal of Natural Products, American Chemical Society, 2003, 66, pp.372-377. 〈10.1021/np020391q〉
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Mohamed Haddad, Tomofumi Miyamoto, Véronique Laurens, Marie-Aleth Lacaille-Dubois. Two new biologically active triterpenoidal saponins acylated with salicylic acid from Albizia adianthifolia. Journal of Natural Products, American Chemical Society, 2003, 66, pp.372-377. 〈10.1021/np020391q〉. 〈ird-01382646〉

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